The present invention relates to an injection-locked oscillator which operates in microwave band, in particular, relates to such an oscillator which is suitable for fabricating the same in an integrated circuit.
An injection-locked oscillator is defined as an oscillator which oscillates with frequency f.sub.0 in free running condition, and is locked or synchronized to frequency f.sub.1 when an external injection signal of frequency f.sub.1 is injected. An injection-locked oscillator is also a frequency multiplier, since the oscillator is locked with high stability and low phase noise oscillation, when a subharmonic frequency which is 1/n, n is an integer, of the free running oscillation frequency is injected.
The operational principle of an injection-locked oscillator is now described.
When an external injection signal is applied to an oscillator circuit which is in free running oscillation condition with frequency f.sub.0, the circuit generates a beat of frequency of difference between the free running oscillation frequency and the external injection signal frequency, and the beat frequency goes to zero so that the free running frequency is locked to the injection frequency. The injection-locking bandwidth .DELTA.f of an injection-locked oscillator is given as follows. ##EQU1## where f.sub.0 is free running oscillation frequency of an oscillator, Q.sub.e is an external Q of the oscillator, P.sub.0 is oscillation output, P.sub.i is power of injection signal. As the value Q.sub.e is smaller and the value P.sub.i is larger, the injection-locking bandwidth is larger. For instance, when P.sub.i :P.sub.0 =1:10, Q.sub.e =1, and f.sub.0 =5 GHz, then, .DELTA.f=1580 MHz, and the frequency stability and the phase noise of an oscillation output are similar to those of an injection signal. The injection locking is also possible when a subharmonic frequency signal (=f.sub.0 /n, n is an integer) is injected to an oscillator, since an oscillator generates harmonic components of frequency f.sub.0 because of non-linear characteristics of the oscillator.
FIG. 6 shows a prior basic injection-locked oscillator, in which a circulator 101 has an oscillation circuit 105 at the first port 102, and the second port 103 for providing an output signal, and the third port 104 for accepting an external injection signal. The arrow in the figure shows the direction of signal flow in the circulator, and the non-reciprocal directivity of the circulator provides an isolation among ports. Oscillator 105 may be implemented by a resonant structure with a negative impedance diode located at a predetermined length from a short circuited plane of a waveguide, or a combination of a strip line used for a resonator.
FIG. 7 shows another prior injection-locked oscillator which is combination of a directional coupler and an amplifier. An injection signal is applied to the first port 112 of the directional coupler 111, and an amplifier 116 is connected between the second port 113 (through port) and the third port 115 (isolation port) of the directional coupler 111. An oscillation output is provided at the fourth port 114 (coupled port) of the directional coupler 111. Solid arrows and dotted arrows in the figure show signal directions viewing at ports 112, and 115, respectively. A directional coupler is not non-reciprocal.
Ports 113 and 115 are connected to amplifier 116 for providing an external feedback loop for amplifier 116. When the phase shift in the feedback loop is 360.degree. at a frequency where the amplifier provides a gain higher than the coupling coefficient, the circuit oscillates at that frequency. When an external signal is applied to port 112, a part of the signal is provided to the amplifier through port 113 so that the oscillation frequency is locked to the injection signal frequency. The oscillation output appears at port 114, but does not appear at port 112 which is an isolation port viewing from the amplifier output.
However, the circuit shown in FIG. 7 has a disadvantage that the operation of the circuit is influenced by an external circuit because the directional coupler is not non-reciprocal between ports 114 and 115.
In a subharmonically injection-locked oscillator, a filter circuit was conventionally used to separate the oscillation frequency path from the signal injection port 112.
A prior injection-locked oscillator employs a non-reciprocal circulator, or a directional coupler to separate an injection port from an oscillation circuit. Therefore, it has a disadvantage which restricts the operation frequency band due to diameter and/or thickness of the ferrite disc, and/or the quarter wavelength lines. The operation frequency band in a prior art is less than 10-50% of the center frequency. Therefore, the locking to a subharmonic frequency (which is f/n, f is oscillation frequency, and n=2, 3, 4 et al) is very difficult, or even if it is possible, a circuit operation is out of design due to the presence of complicated elements in a non-reciprocal circuit.
Further, a circuit using a circulator is impossible to be implemented the same in an integrated circuit, because of the use of a ferrite component.
A circuit using a directional coupler is too large in size to be implemented as an integrated circuit at frequencies less than 10 GHz because size of the directional coupler is anti-proportional to frequency. Further, a directional coupler type injection-locked oscillator has a disadvantage that it is influenced by an external circuit, because the oscillation loop is not electrically isolated from an injection signal input and/or an oscillation output.
A prior injection-locked oscillator which is locked to subharmonic frequencies has a filter circuit for isolating a signal injection port from an oscillation frequency paths, and therefore, it has the disadvantage that a subharmonic coefficient n must be restricted, and further it is impossible to lock to the fundamental frequency (n=1).